1. Field of the Invention
The present invention relates to an apparatus for measuring the lung characteristics of a patient. It more particularly addresses a portable spirometer for measuring various parameters which are necessary to the diagnostic of doctors, such as the peak-flow, the maximum exhaled volume per second, etc.
The peak-flow is often measured for people who suffer from asthma, bronchitis, mucoviscidosis, or who have been lung transplanted. Indeed, especially for those suffering from asthma, this measure allows the anticipation of an asthma crisis and, during treatment, allows the dosage of medicine intake.
It is thus desirable that the patients can regularly analyze their peak-flow, chart their characteristics and communicate these to their doctor or directly use them in a way prescribed by their doctor.
2. Description of the State of the Art
A known spirometer is of mechanical type, in which the breath of a patient acts on a spring biased piston. These mechanical devices have the advantage of being small and cheap but their main drawback is that they are little reliable, inaccurate and inconstant.
To realize an electronic flowmeter, it is known to use a Venturi flow sensor. Such a sensor comprises a tube with a narrow part. When a patient blows into the tube, a pressure difference establishes on both sides of the narrow part which is substantially proportional to the flow in a certain measuring range depending on the dimensions and the shape of the narrow part. This difference is measured by a differential pressure sensor connected through conduits to regions situated on either side of the narrow part.
A specific problem in the use of a Venturi flow sensor in a spirometer is connected to the fact that the spirometer must be regularly cleaned and sterilized, eventually after each use. In particular, the development of germs coming from saliva or other expectoration of the patient should be avoided in the tube and in the conduits which arrive to the sensor. Moreover, the cleaning is not easy because of the small functional passage at the narrow part and because of unavoidable corners and angles which are difficult to reach and which accumulate dirt.
U.S. Pat. No. 5,137,026 describes a portable electronic spirometer using such a Venturi flow sensor. The tube is integral with the casing of the device. To avoid dirt from going into the conduits which arrive to the pressure sensor, particular filters are placed in these conduits where they open into the tube. These filters must be liquid-tight and permeable to air. To clean and sterilize the tube, the whole device must be plunged in an antiseptic solution. Therefore, the device must be perfectly water-tight, which increases its price substantially.